What does the icon to the left of the object name in the outliner mean?
If you examine the "add objects" menu, visible by using the SHIFT-Akey combination, or the  Spacebar > Akey sequence, you will see that the icon to the left of "stem final copy", and "stem_final" in the excerpt of the image of the outliner panel you link to is used for one of three object types: an empty object, a force field, and a group instance. None of the three is a mesh, and so cannot be part of a boolean operation, as these take two mesh data. 

You can create something equivalent to an empty by adding a mesh object, and in edit mode selecting and then deleting all of its vertices, but I don't see in the Blender Manual any information on converting an empty, force field, or group instance to a mesh.